Neutral sixth

A neutral sixth is a musical interval wider than a minor sixth playⓘ but narrower than a major sixth playⓘ. Three distinct intervals may be termed neutral sixths:

  • The undecimal neutral sixth has a ratio of 18:11 between the frequencies of the two tones, or about 852.59 cents. playⓘ
  • A tridecimal neutral sixth has a ratio of 13:8 between the frequencies of the two tones, or about 840.53 cents.[3] This is the smallest neutral sixth, and occurs infrequently in music, as little music utilizes the 13th harmonic. playⓘ
  • An equal-tempered neutral sixth is 850 cents, a hair narrower than the 18:11 ratio. It is an equal-tempered quarter tone exactly halfway between the equal-tempered minor and major sixths, and half of an equal-tempered perfect eleventh (octave plus fourth). playⓘ
Neutral sixth
Inverseneutral third
Name
Other names-
Abbreviationn6
Size
Semitones~8½
Interval class~3½
Just interval18:11[1] or 13:8[2]
Cents
Equal temperamentN/A
24 equal temperament850
Just intonation853 or 841
Neutral sixth on C Playⓘ.

These intervals are all within about 12 cents of each other and are difficult for most people to distinguish. Neutral sixths are roughly a quarter tone sharp from 12 equal temperament (12-ET) minor sixths and a quarter tone flat from 12-ET major sixths. In just intonation, as well as in tunings such as 31-ET, 41-ET, or 72-ET, which more closely approximate just intonation, the intervals are closer together.

A neutral sixth can be formed by subtracting a neutral second from a minor seventh. Based on its positioning in the harmonic series, the undecimal neutral sixth implies a root one minor seventh above the higher of the two notes.

Thirteenth harmonic
Thirteenth harmonic on C. Playⓘ

The pitch ratio 13:8 (840.53 cents) is the ratio of the thirteenth harmonic and is notated in Ben Johnston's system as A13♭. In 24-ET is approximated by Ahalf flat. This note is often corrected to a just or Pythagorean ratio on the natural horn, but the pure thirteenth harmonic was used in pieces including Britten's Serenade for tenor, horn and strings.[4]

See also

References
  1. Haluska, Jan (2003). The Mathematical Theory of Tone Systems, p.xxiv. ISBN 0-8247-4714-3. Undecimal neutral sixth.
  2. Haluska (2003), p.xxiii. Tridecimal neutral sixth.
  3. Jan Haluska, The Mathematical Theory of Tone Systems, CRC (2004).
  4. Fauvel, John; Flood, Raymond; and Wilson, Robin J. (2006). Music And Mathematics, p.21-22. ISBN 9780199298938.
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